1. Field of the Invention
The present invention relates to a coated substrate with high reflectance. It is especially concerned with transparent glass substrates bearing a coating of oxides of tin and antimony and with the use of such substrates in exterior glazing panels for buildings.
2. Description of the Related Art
Although architects seeking glazing panels for use in buildings have traditionally tended to favour panels with low levels of reflection, a changing perception of the aesthetic appeal has led to increasing demands for panels with higher levels of reflection but without the glare as viewed from outside which is associated with very high levels of reflection. The panels may also be required to have other qualities such as providing protection for occupants of the building against solar radiation and the associated overheating (solar screening properties).
The panels comprise at least one sheet of a transparent substrate material, typically soda-lime glass, with a thin coating on one or more of the sheet faces to modify the optical and physical properties of the sheet and the panel as a whole. A huge variety of prior proposals have been made for the coating, according to the specific properties sought. The coating may comprise a stack of several discrete layers chosen with appropriate compositions and thicknesses to complement their respective effects. A persistent problem in choosing the respective layers is that a layer adopted for one purpose may adversely change the effect of other layers.
Tin oxide (SnO2) has been widely used as a coating material, often in combination with other metal oxides. Coatings comprising tin oxide with a small proportion of antimony oxide have proved especially attractive.
Our GB patent 1455148 teaches a method for pyrolytically forming a coating of one or more oxides (e.g. ZrO2, SnO2, Sb2O3, TiO2, Co3O4, Cr2O3, SiO2) on a substrate, primarily by spraying compounds of a metal or silicon, so as to modify the light transmission and/or light reflection of the substrate. Our GB patent 2078213, which relates to a method for pyrolytically forming a coating by two separate sprays to achieve high rates of coating build-up, discloses tin oxide coatings doped with fluorine or antimony. Our GB patent 2200139 relates to forming a pyrolytic tin oxide coating from a precursor containing at least two additives such as oxidising agents, sources of fluorine and sources of metal.
The use of a tin oxide coating with a small proportion of antimony oxide has been found to offer several advantageous combinations of optical and energy properties. Our GB patent applications 2302101 ('101) and 2302102 ('102) describe anti-solar glazing panels comprising a pyrolytic coating layer of oxides of tin and antimony in which the Sb/Sn molar ratio is from 0.01 to 0.5. The '101 coating is applied by liquid spray and has a thickness of at least 400 nm, a luminous transmittance of less than 35% and a selectivity of at least 1.3. The '102 coating is applied by chemical vapour deposition (CVD) and has a solar factor below 70%.
The use of pyrolysis to form a coating on a substrate generally has the advantage of producing a hard coating with durable abrasion-resistant and corrosion-resistant properties. It is believed that this is due in particular to the fact the process involves deposition of coating material on to a substrate which is hot. Pyrolysis is also generally cheaper than alternative coating processes such as sputtering, particularly in terms of the investment in plant.
Properties of the coated substrate discussed herein are based on the standard definitions of the International Commission on Illumination—Commission Internationale de l'Eclairage (“CIE”). The illuminant for the tests was illuminant C, which represents average daylight having a colour temperature of 6700 K and is especially useful for evaluating the optical properties of glass intended for use in buildings.
The “luminous transmittance” (TL) is the luminous flux transmitted through a substrate as a percentage of the incident luminous flux.
The “luminous reflectance” (RL) is the luminous flux reflected from a substrate as a percentage of the incident luminous flux.
The “purity” (p) of the colour of the substrate refers to the excitation purity in transmission or reflection.
The “dominant wavelength” (λD) is the peak wavelength in the transmitted or reflected range.
The “solar factor” (FS), referring to the transmission of total incident solar radiation through the coated substrate, is the sum of the total energy directly transmitted (TE) and the energy which is absorbed and re-radiated on the side of the coated substrate away from the energy source, as a proportion of the total incident radiant energy.
The “selectivity” of a coated substrate for use in a building glazing panel is the ratio of the luminous transmittance to the solar factor (TL/FS).
It is an object of the present invention to provide a pyrolytically formed coating on a substrate to impart solar screening properties and a high reflectance to the substrate.